1. Field of the Invention
This invention relates to magnetic flux sensors and methods. This invention also relates to sensor methods and systems that utilize magnetic flux sensors to acquire information pertinent to another ultimate parameter of interest, such as current.
2. Description of Related Art
A common problem with magnetic devices is that there is often no practical way of knowing how much magnetic flux is present in the device. This parameter is of obvious interest in any magnetics application, but is of particular interest in applications-where the magnetic material that carries the magnetic flux is liable to become saturated.
Devices that in some way utilize magnetic flux are common and have been employed in a diverse array of applications. For example, magnetic devices such as transformers are commonly used by utilities and in various household and industrial applications to convert power sources from one voltage level to another voltage level. Another type of transformer is a current transformer, which is a widely employed device for performing current measurements. Magnetic devices also include electromechanical devices such as relays, electromagnetic contactors, electric motors, and electric generators. Relays and electromagnetic contactors are used to control whether a particular electrical connection is opened or closed. Electric motors and electric generators are used to convert electrical power into mechanical power and vice versa. Numerous other magnetic devices also exist.
Current transformers provide an especially good example of the problem. A current transformer comprises primary and secondary windings that are wound about a transformer core. A primary current I1 flows through the primary winding and induces a magnetic flux which flows through the transformer core. The magnetic flux in turn induces a secondary current I2 in the secondary winding. For a linear (unsaturated) current transformer, the primary current I1 is related to the secondary current I2 by the following relationship:I1N1=I2N2  (1)
Therefore, since the parameters N1 and N2 are known (N1 and N2 are the number of turns of the primary and secondary windings, respectively), the primary current I1 may be measured indirectly by measuring the secondary current I2. The secondary current I2 may be measured by placing a burden resistor across the secondary winding, and measuring a voltage V2 developed across the burden resistor as a result of the secondary current I2:                               I          1                =                                            V              2                                      R              b                                ⁢                      (                                          N                2                                            N                1                                      )                                              (        2        )            where Rb is the resistance of the burden resistor. In short, therefore, the primary current I1 may be determined by measuring the voltage V2.
The voltage V2 that is developed across the secondary winding is related to the net magnetic flux Φ in the following manner:                               V          2                =                              N            2                    ⁢                                           ⁢                                    ⅆ              Φ                                      ⅆ              t                                                          (        3        )            (Although Eq. (3) is sometimes written with a minus sign before the right-hand term, a minus sign is not used herein.) In saturation, since the net magnetic flux Φ stays at a constant saturated level, there are no time varying changes in the net magnetic flux Φ. As a result, there is no voltage developed across the secondary winding (V2=0) and there is no secondary current that flows through the secondary winding (I2=0). This is true even though current continues to flow in the primary winding (I1≠0). It is therefore apparent that, in saturation, Eqs. (1) and (2) do not apply and the primary current I1 cannot be measured.
Typically, saturation can be avoided by only measuring currents above a certain frequency and below a certain magnitude, these operational limits being determined by the construction of the current transformer. However, sometimes low frequency components appear unexpectedly in the primary current, causing the current transformer to go into saturation. Therefore, knowing the amount of magnetic flux in the transformer core would be highly advantageous, because it would provide an opportunity to take measures to counteract the low frequency components that would otherwise cause the transformer core to saturate. Indeed, it would be even more advantageous if those low frequency could not only be counteracted, but measured as well.
This same general phenomenon also exists with respect to other magnetic devices. For example, synchronous electric motors operate through the creation of a magnetic field that rotates in synchronism with the rotor. The rotating magnetic field is generated by providing the stator with sinusoidal drive current. However, given that the drive current is often electronically-generated, it is possible that DC and/or other low frequency current components can “creep into” the drive current, causing the magnetic material in the motor to tend toward saturation. Such current components can occur, for example, if the switching transistors used to generate the sinusoidal excitation current are not perfectly matched. Magnetic losses in motors often help avoid saturation, but low frequency current components nevertheless at least cause the motor to operate less efficiently.
Even ignoring the problem of saturation and low frequency current components, it is often desirable for other reasons to monitor the magnetic flux in a magnetic device. The provision of a rotating magnetic field is a fundamental aspect motor control. Typically, however, the magnetic flux in the motor is not directly measured but rather is assumed to have a certain value (or distribution of values) based on the known current that is applied to the motor. Being able to directly measure the magnetic flux in an electric motor would provide an opportunity for better, more-efficient control of the motor.
Likewise, for electromagnetic contactors, or for other devices in which an electromagnetic field provides an actuating force for moving a mechanical substructure, a direct measure of the magnetic flux would allow the actuating motion to be controlled more precisely. This could be used to improve operation of the device or to effect other desirable results, such as extending the life of the device.
Magnetic flux sensors have previously been provided. For example, current transformers are one type of magnetic flux sensor, i.e., because a current transformer operates by having a secondary winding that senses magnetic flux in the core of the transformer. Conventional current transformers, however, are not well-suited to measuring low frequency flux components for the reasons previously described.
Another type of magnetic flux sensor is the Hall-effect sensor. When a conductor carrying a current is placed in a magnetic field, a voltage is created across the conductor in a direction that is perpendicular to both the direction of the magnetic field and the direction of current flow. This well known phenomenon is referred to as the “Hall-effect,” and is the operating principle for Hall-effect sensors. Magnetic flux sensors that operate based on the Hall-effect have been employed in a diverse array of applications, such as current sensors.
A primary disadvantage of Hall-effect sensors, however, is that they must be placed in the magnetic path, which usually requires that a gap be made in the flux-carrying material. Given the extremely low permeability of air (approximately 1.0) as compared to most core materials (in the range of 104 to 105 depending on the material used and operating conditions), the insertion of an air gap, however small, has a dramatic and usually undesirable effect on the magnetic characteristics of the system. For example, conventional current sensors that use Hall-effect devices have significantly poorer resolution and accuracy than current transformers over those operating ranges in which current transformers do not saturate.
Therefore, what is needed is an improved method and system for magnetic flux sensing. What is also needed is an improved method and system for flux sensing that is capable of operating in the presence of low frequency flux components, and even more preferably capable of measuring those low frequency flux components.